Update mIsFaceEnrollable after face enrollment

Since face enrollment can be launched first, the mIsFaceEnrollable may
be changed during the multiple enrollment process. We should update this
whenever a face is enrolled.

Bug: 370940762
Test: 1. no face and fingerprint enrolled
      2. click the "Face" in "Device unlock" to launch the multiple
	 enrollments
      3. enroll a face and in fingerprint enrollment process, trigger
	 back key
      4. should return to "Device unlock" page
Flag: com.android.settings.flags.biometrics_onboarding_education
Change-Id: I21ed3d21cd81f1d5c121111e2353cf4a9eb5bce6